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Practical Directions for 
Marking and Adjusting 

THE CORLISS 

VALVE GEAR 



i 



With One and Two Eccentrics 



By H. H. KELLEY 



W. R. C. SMITH PUBLISHING CO. 



Atlanta, 



1910 



ieorgia 



^_ 



3 FOR $1.00 




The A. B. C. of the 
Indicator. 

r. M H is.-iley 
This i com- 

■ 
an 1 1) 'i it and putt 

lilng it. 
when conin 
up a reducing i 

iliagram and when putting 
an Indicator away after using. It 
is nisi' reading" a dla- 

declpherlng puzzling cards 
and figuring i from 

the 'l 




The Engine Room 
Instructor. 

By H. II. Kelley 

This book will tell you what 

ou don't know it all. 

IV;i.-hi i urowi l)OW ti 



lMKton Valves, a Link Motion and 
a Dupii i 

■ I ' ! . i 

of Joints and the I" 

i E3 to 1 .nd the 

llorsfj.ov f 1 v. 

and Win.] H 

i I the Powi i I ■ 

, • ; lllMtl 

- Binding. 



HOW TO GET THEM: 

this ad 1 iscrlptlon to 

The Southern Engineer 

n power plant paper — an operating engineer's 
ireful rending of Its pages from month to month will 

.tains no pet theories nor useless calculations; 

bring i "-h month plain, practicnl articles written 

charge of plnnts or nre oper- 



Southern Engineer 

Atlanta, Georgia. Agents Wanted 



ADJUSTING THE CORLISS VALVE GEAR. 



Single Eccentric Gear. 

IN preparing the following instructions for setting and adjust- 
ing the standard type of Corliss valve gear, both single 
and double eccentric, it has been assumed that the reader 
is familiar with the construction of the standard form of Corliss 
gear, and with the operation of the gear when it is in proper 
working order. Having that much information to start with, 
the reader will find little difficulty in setting almost any make 
of Corliss engine valve gear, provided he will follow the instruc- 
tions carefully and will carry on the work, step by step, as 
directed. In the following instructions it is assumed that the 
eccentric is secured to the shaft by means of set screws, in which 
case the proper position of the eccentric must be determined. 
Thus the instructions will cover nearly all cases of failure of the 
gear and enable the engineer to substitute new parts and to 
set the entire gear correctly without the aid of marks usually 
put on the several parts by the builders. 
Lead and Tools. 
The amount of lead mentioned in this pamphlet is 1-32 of an 
inch, which is sufficient for all sizes of cylinders ordinarily met 
with in practice, excepting low pressure cylinders of compound 
engines, which usually require 1-16 of an inch lead. The tools 
required for marking and setting the gear are: Steel scriber, 
straight-edge, flexible scale 4 or 6 inches long, or a pair of di- 
viders, 2-foot rule, sharp cold chisel and a hammer. 
Putting the Gear in Order. 
Before attempting to set the eccentric or to adjust the position 
of the wristplate and valves of the Corliss engine, the valve 
gear should be put into good working order. By this is meant 
that all unnecessary lost motion should be taken up, and if any 
brasses need renewing or need shimming up, as it is called, this 
should be done ; and all the joints adjusted and properly lubri- 
cated before attempting to adjust the valves, otherwise a sub- 
sequent adjustment of the valve gear might render the adjust- 



Copyright 1910, by H. H. Kelley. 






y 



& 



ment of the valves very inaccurate, thus necessitating going 
over a considerable portion of the work a second time. 

The gear should first be cleaned, and the dashpots caused to 
seat properly, provided they do not already do so. The die 
plates on the hooks and catchblocks should be either renewed or 
turned around so as to offer true, sharp contact edges. The 
lap or the extent of engagement between the hooks and catch- 
blocks should be kept down to as near 1-16 of an inch as possi- 
ble, and both hooks should have the same lap or engagement 
with the catchblocks. The safety device should be placed so 
that the hooks will normally engage the catchblocks when the 
engine is not in motion. Observing these minor details previous 
to attempting to set the Corliss valve gear will save much time 
and annoyance, and in the end will insure a much more accurate 
job, and consequently a better working engine. 

Equalizing the Motion. 

The first work in setting the Corliss valve gear is to equalize 
the movements of the several parts. In order that the same 
quantity of steam may be admitted to and exhausted from each 
end of the cylinder at the proper time, the valves must be given 
equal travel, that is, both steam valves should open at the same 
time with reference to the position of the piston, and the valves 
should travel equal distances in opening the ports. The exhaust 
may have equal travel the valve gear must be so adjusted that it 
will have equal travel with reference to a fixed point. This 
point is the plumb line — that is to say, the driving pin in the 
wristplate should move the same distance on both sides of the 
plumb line when the line is suspended in front of the center 
of the wristplate stud or bearing. 

As the reach rod, which connects the wristplate to the rocker 
arm, is seldom adjustable in length, it follows that if the rocker 
arm be placed in a vertical position the wristplate will occupy 
its central position, or its position of mid-travel. As the rocker 
arm is easier to plumb than the wristplate, on account of the 
pins being nearer the line vertically, the rocker arm is first 
placed in its central position, as shown in Fig. 1. The full side 
of the eccentric will now occupy one of the quarters. Pay no 



3 



attention to the position of the eccentric further than to see that 
the center of the full side of the eccentric is about vertical. No 
attention need be paid to the crank or piston, these will be at- 
tended to as the work progresses and each brought into the 
proper position. 

Marking the Wristplate. 
While the rocker arm is in the central position, take a straight- 
edge and scriber and draw a line on the hub of the wristplate, 
extending it to the edge of the wristplate support, which usually 
is flush with the hub of the wristplate. This line will, of course, 
be divided into two parts, one part being on the wristplate hub, 
which part of the line we shall call B, Fig. 1 A, and the part 
on the support is designated A. In order that the wristplate 
may impart equal motion to the two steam valves and equal 
motion to the two exhaust valves, it is evident that the line, B, 
will have to move the same distance both ways from line, A. 
The movement of the wristplate is limited by the eccentric, so 
that when the eccentric reaches the extreme position, the wrist- 
plate will have been moved to the end of its travel. Therefore, 
to measure the travel of the wristplate, move the eccentric in 
either direction as far as it will go, as shown in Fig. 1 B, or 
until it ceases to move the wristplate. With the scriber and 
straight-edge draw a very light line C, on the wristplate hub 
and in line with the one marked A. Measure the distance with 
a flexible scale or a pair of dividers, between lines B and C. 
Then have the eccentric turned to the opposite extreme position 
as shown in Fig. 1 C, and draw another line Z>, on the wrist- 
plate hub. With the dividers try the distance between lines B 
and D. If the travel of the wristplate is equal on both sides of 
the line A, it is evident that the distances between the lines B 
and C, and between the lines B and D, must be the same. If 
one of these distances is found to be greater than the other, the 
eccentric rod should be adjusted so as to make the shorter dis- 
tance longer; the exact amount to move the eccentric rod will 
be whatever is necessary to move the wristplate an amount 
equal to one-half the difference between the distances BC and 
BD. Then, after trying these distances by moving the eccen- 
tric to the extreme positions, scribe the lines deeper so that the 



corrected positions may be obtained at other times. Both the 
rocker arm and the wristplate will then have equal travel on 
both sides of the plumb line and consequently will be able to 
impart equal motions to the valves. 

Equalizing the Position of the Valves. 
The next step is to equalize the positions of the valves. The 
positions of the steam valves, when hooked up, and of the ex- 
haust valves, can only be equal or the same when the wristplate 
is in its midtravel position. That position is obtained by mov- 
ing the eccentric until the lines A and B, Fig. 1 A, are exactly 
in line. It is well to secure the wristplate in this position, be- 
cause when making adjustments of the other parts of the gear 
it is oftentimes quite easy to move the entire gear, especially 
when the eccentric is loose on the shaft. The gear may be locked 
by tightening the set screw in the eccentric or by putting in 
a thin washer between the wristplate hub and the washer which 
holds it on the stud, and then tightening the nut. One valve 
should now be partly raised — that is, the hook at one end of the 
cylinder will have engaged the steam valve crank-arm and will 
have raised it a certain distance, and with it the dashpot plunger. 
The valve arm, which is now disengaged from the hook, should 
be raised and hooked on so that both steam valves will occupy 
the same position at the same time, as shown in Fig. 1. 

Marking the Valves. 

If, upon removing the front bonnets from the valve chests, 
no marks are seen on the ends of the valves and on the valve 
seats, take the steam valves out, one at a time, and mark them, 
replacing one valve before removing another, so as not to get 
the valves in the wrong chests. Take a try-square and scriber, 
and placing the blade parallel to the working edge of the valve, 
as shown in Fig. 2, draw a fine line to the end of the valve; 
then draw a radial line across the end of the valve to meet it, 
as shown in the same figure. After checking up to see that the 
lines are correctly located, take a sharp chisel and make the 
radial line deeper near the edge of the valve. 

In case of multiported valves, select the working edge that 
is nearer the bottom of the chest, because it is easier to measure 



its movements when adjusting the valve. Be sure that corres- 
ponding edges are selected on both valves, as shown in the upper 
view in Fig. 1. While each valve is out, take a straight-edge and 
scriber and draw a line on the valve seat in line with the working 
edge of the port that corresponds to the working edge marked on 




WORKING COGE LAP 

FIG. 2. SHOWING LAP OF VALVE, METHOD OF MARKING AND 
HOOK CLEARANCE. 

the end of the valve. Make the line long enough so that it can be 
seen when the valve is replaced in the chest. Mark all the valves 
and valve seats in this manner. It will no doubt be seen that 
when the valves are in their proper places in the chest, the work- 
ing edge of both the valves and ports can be easily and accurately 
located by means of the marks and the exact position of the 
valves determined. 

Lap of Valves. 

When the valves are all in place the working edges of the 
steam valves should lap over the working edges of the ports, as 
shown in Fig. 2, the amount of the lap being indicated by the 
distance between the mark on the end of the valve and the mark 
on the valve seat, as shown in the upper view in Fig. 1, which 
shows the ends of both of the valves, which are shown in section 
below. This distance representing the lap varies with different 
sized engines. The amount of the lap to be given the steam 
valves will be found in the accompanying table. 

Take the distance representing the lap between the points of 
the dividers and with one point on the line on the valve seat and 
the other point lying toward the middle of the cylinder, have the 
valve moved until the line on the valve reaches the point of the 



TABLE SHOWING LAP AND TRIAL COMPRESSION, 



. SIZE OF 
CYLINDER 


Lap of Steam 
Valvei, Wrist- 
plate at 
Midtravel 


TRIAL COMPRESSION 


Minimum 


Non-Condens- 
ing Single 
Cyl. 


Comp. Non- 
Cond. L. P. 

Cyl 


Condensing 


Steam Valves. 
Double Eccen- 
tric. 




Inch 








Inch 


8x24 
10x24 


5—32 


1 1-2 ins. 
1 1-2 


3 ins. 
3 


4 ins. 
4 


5-32 


12x30 




2 


3 3-4 


5 




12x36 


3—16 


2 1-4 


4 1-2 


6 


3—16 


14x36 




2 1-4 


4 1-2 


6 




14X42 


1—4 


2 3-4 


5 1-4 


7 


3—16 


14x48 




3 


6 


8 




16x32 




2 


4 


5 




16x36 




2 1-4 


4 1-2 


6 




16x42 


5—16 


2 3-4 


5 1-4 


7 


1 — 4 


16x48 




3 


6 


8 




18x36 




2 1-4 


4 1-2 


6 




18x42 




2 3-4 


5 1-4 


7 




18x48 




3 


6 


8 




20x42 


3—8 


2 3-4 


5 1-4 


7 


1—4 


20x48 




3 


6 


8 




20x60 




3 3-4 


7 1-2 


10 




22x42 
22x48 


13—32 


2 3-4 
3 


5 1-4 
6 
7 1-2 


7 

8 
10 


1-4 


22x60 




3 8-4 




24x42 




2 3-4 


5 1-4 


7 




24x54 




3 1-2 


6 3-4 


9 




24x60 
26x48 




3 3-4 
3 


7 1-2 

6 

7 1-2 

6 


10 
8 

10 
8 

10 




26x60 
28x48 


7—16 


3 3-4 
3 


9—32 


28x60 




3 8-4 


7 1-2 




28x72 




4 1-2 


9 


12 




30x48 




s 


6 

7 1-2 
9 


3 
10 

12 




30x60 
30x72 


15—32 


8 3-4 

4 i-2 


5—16 


31x48 
32x60 
32x72 




3 

3 3-4 

4 1-2 


6 

7 1-2 
9 


8 
10 
12 




84x48 


1—2 


3 


6 

7 1-2 

9 


8 
10 
12 


5—16 


34x60 
34x72 




3 3-4 

4 1-2 




36x48 




3 


6 

7 1-2 

9 


8 

10 
12 




36x60 
36x72 


17-82 


3 3-4 

4 1-2 


5—16 


88x60 




3 3-4 


7 1-2 


10 




40x48 




3 


6 


s 




40x60 
40x72 




3 3-4 

4 1-2 


7 1-2 
9 


10 

12 




40x84 
42x48 


9-16 


5 1-4 
3 


10 1-2 

6 


14 

g 


3—8 


42x60 




3 3-4 


7 1-2 


10 




42x72 




4 1-2 


9 


12 




44x48 




3 


6 


g 




44x60 




3 3-4 


7 1-2 


10 




44x72 


5—8 


4 1-2 


9 


12 


3—8 


46x60 




3 3-4 


7 1-2 


10 




48x72 




4 1-2 


9 


12 




48x60 

48x72 


11-16 


3 3-4 

4 1-2 


7 1-2 

9 1 


10 
12 


7—16 



8 

dividers. The valve is moved by turning the right-and-left ad- 
justment in the radial rod connecting the valve with the wrist- 
plate. As soon as the valve has been moved into the proper posi- 
tion, tighten the jam nuts in the right-and-left adjustment in the 
radial rod. Adjust the valve at the opposite end of the cylinder 
in precisely the same manner. The lines on the backs of the 
valves and on the seats will then appear in the position shown 
in the upper view in Fig. 1. 

The exhaust valves should be adjusted so that the marks on 
the ends of the valves and on the valve seats are nearly in line, 
allowing not more than one-sixteenth of an inch lap. See lower 
view, Fig. 1. The right-and-left adjustment in the radial rods of 
the exhaust valves may be loosened as further adjustment of 
these rods will be necessary. Now remove the washer from the 
wristplate hub or loosen the set screws in the eccentric, depending 
upon how the gear has been locked in position, thus releasing the 
gear so that the eccentric can be turned on the shaft. Turn the 
eccentric to one of its extreme positions. See that the corres- 
ponding dashpot plunger is well seated. The clearance, 0, Fig. 2, 
between the die on the hook and the die on the catchblock at- 
tached to the crankarm on the valve stem, should be about 1-32 
of on inch. The clearance is adjusted by lengthening or shorten- 
ing the dashpot rod if need be, after which the jamnuts should be 
carefully tightened. Turn the eccentric to the opposite extreme 
position and adjust the clearance at the opposite hook in pre- 
cisely the same manner. 

Setting the Eccentric. 

The next step is to set the eccentric with reference to the 
crank. Place the crank on the exact dead center — preferably the 
one nearest the cylinder. Turn the full part of the eccentric to 
the corresponding position and so that the top of the rocker arm 
will have been moved as far as possible toward the head end of 
the cylinder, which will bring the head end hook into position to 
engage the head end catchblock on the steam valve crank-arm. 
Have the eccentric turned in the same direction in which the 
crank is to turn until the valve at the head end of the cylinder 
has opened the port 1-32 of an inch, as shown in Fig. 3 ; which 
opening will be indicated by the lines on the end of the valve and 



10 



on the valve seat. These lines shonld then be 1-32 of an inch 
apart, as shown at the left-hand side of the npper view in Fig 3. 
Have the eccentric tightened to the shaft, when the eccentric 
will have been properly set. It is important that the lead be the 
same at both ends of the stroke, therefore, to ascertain whether 
the valve at the crank end gives 1-32 of an inch lead, have the 
crank tnrned to the crank-end dead center and again measure the 
lead If it is more or less than 1-32 of an inch, move the valve 
slightly by adjusting the right-and-left connection in the radial 

rods. 

Equalizing the Lead on Large Engines. 

If the engine is a large one or is of the compound type, it will 
not be convenient to turn the crank from one dead center to the 
other to see whether the lead is equal. In that case, after setting 
the eccentric as shown in Fig. 3, measure the distance between 
line A and line B, Fig. 1 B, which is the measure of the full 
movement of the wristplate out of a central position. Eaise the 
reach rod from the driving pin in the wristplate, and by means of 
the starting bar, have the wristplate moved in the opposite direc- 
tion until the line B is exactly the same distance on the other side 
of line A. It will be understood that the wristplate will now 
occupy the same position that it would if the crank had been 
turned to the crank-end dead center, and consequently, the lead 
may be thus adjusted at the crank end without turning the crank. 
If the re-adjustment for lead amounts to 1-32 of an inch, the gear 
should be put into its extreme position and the clearance 0, 
Fig. 2, again adjusted, if need be, by means of the dashpot rod. 
Adjusting the Exhaust Valves. 
The next step is to adjust the exhaust valves so that they 
will give approximately the proper compression. First look in 
the table of Trial Compression and opposite the given size of 
engine will be found the point in the return stroke at which the 
exhaust valve should close the port. If the crosshead overrides 
the guides at the ends of the stroke, it will be desirable to place 
the crank on the dead centers and then mark the position of the 
end of the crosshead on the guide or frame, as at E, Fig. 4. Be- 
ginning at this line, measure off the distance given in the table 



11 




/^ 



12 



and make another line, F. Do this at the opposite end of the 
guides, as shown at G and H, Fig. 4. 

Starting with the crosshead at one end of the stroke — say at 
the end nearest the cylinder, have the crank turned in the same 
direction in which the engine is to run until the end of the cross- 
head reaches the line F near the end of the stroke, as shown in 
Fig. 4. The exhaust valve that corresponds to this end of the 
stroke, or that which is nearest the piston — in this case the crank- 
end valve — should have just closed the port. The position of the 
valve can be seen by means of the mark on the end of the valve 
and the mark on the valve seat; these should be exactly in line 
with each other, but if they are not in line, the valve may be 
moved one way or the other by means of the right-and-left ad- 
justment in the radial rod which connects the valve with the 
wristplate. After bringing these two lines in line with each other, 
have the crank turned in the same direction as before until the 
cross head reaches the other line H, near the opposite end of the 
guides, as shown in Fig. 5. The opposite valve — in this case, the 
head valve — should now have just closed the port, and conse- 
quently, the lines on the end of the valve and on the valve seat 
should be exactly in line with each other. This valve may be 
adjusted the same as the other one, if need be, after which the 
jamnuts in both the radial rods connected to the wristplate should 
be tightened and the exhaust chest bonnets put on. The steam 
valve bonnets also may now be put on, if desired, because the 
lines on the valves and seats will be of no further use in making* 
the remainder of the adjustments. 

Eegulating the Governor for Safety. 

The next step is to see that the governor will prevent the 
hooks from engaging the catchblocks on the steam valve crank- 
arms when the governor balls are in their lowest position and 
when they are in their highest position. Take out the collar, cam 
or pin, or whatever may be employed to prevent the governor 
from reaching its lowest position, and let the balls go down as 
far as they will go. Turn the crank to one of the dead centers, 
then examine the hook corresponding to that dead center. If it 



13 

has raised the steam valve crank-arm, the governor rod connected 
to the knock-off cam or trip plate at that end of the cylinder 
should be adjusted until the crank-arm is released from the hook. 
Put in the starting bar and raise the reach rod from the driving 
pin in the wristplate, then turn the wristplate to the opposite ex- 
treme position, which will be indicated by the marks AC or AD 
on the wristplate hub and the support, being in line with each 
other. Now examine the other hook, and if need be, adjust the 
governor rod at that end so that the hook will just fail to engage 
the catchblock.. 

Next raise the governor balls to their highest position and 
examine both hooks to see that they do not engage the catch- 
blocks on the crank-arms. If one of the hooks only should engage 
the catchblocks, examine the trip plate to see that the projection 
which trips the hook is not worn or loose. If it is all right, then 
adjust the governor rod at that end so that the hook will just 
clear the catchblock. In event both hooks engage the catchblocks, 
raise the upper collar on the governor spindle slightly, if practic- 
able, after raising the balls, and again examine the hooks, which 
will now clear the catchblocks. If the collar is fixed and cannot 
be raised, then adjust the governor rods so as to prevent the hooks 
from engaging the catchblocks. It should be stated in this con- 
nection that when the hooks fail to engage the catchblocks when 
the governor balls are in their lowest position, the hooks will also 
fail to engage the catchblocks when the balls are in their highest 
position, except when the trip plates are loose or have become 
badly worn and need replacing. It is well, therefore, to try the 
hooks with the balls in both the highest and lowest positions and 
to have the hooks clear by considerable in order to allow for 
some adjustment of the trip plate when equalizing the cutoff. 
When the hooks just clear the catchblocks, when the balls are in 
their lowest position a very slight adjustment of the governor 
rods, as a usual thing, will cause them to clear the catchblocks 
when the balls are in their highest position. 

Equalizing the Cutoff. 

The next and last step is to adjust the trip plates or knockoff 
cams so that they will produce an equal cutoff on both strokes. 



14 

The average point of cutoff in single eccentric Corliss engines 
when properly loaded is at about one-quarter stroke. First divide 
the length of the stroke expressed in inches by 4, and beginning 
at the lines G and E, Fig. 4, on the guide or frame, representing 
the ends of the crosshead travel, measure off one-quarter of the 
stroke and make a fine line on the guide as at J and K, Figs. 4 
and 5. Be careful to obliterate the lines H and F, for the time 
being, since the latter lines show where the exhaust valve should 
close; otherwise there will be two lines rather close together, 
which may prove to be confusing. 

After measuring off one-quarter stroke from both ends of the 
crosshead travel (not the ends of the guide) and properly mark- 
ing the guide at these points, as shown at J and E, Figs. 4 and 5, 
have the crank turned in the same direction in which the engine 
is to run until the end of the crosshead nearest the end of the 
guide comes to the line representing one-fourth stroke. It is 
evident that if the cutoff is to occur at this point in the stroke, 
the steam valve corresponding to this end of the cylinder should 
be released from the hook at this point ; therefore, raise the gov- 
ernor balls slowly until this valve is released. Block the governor 
in. exactly this position, have the crank turned in the same direc- 
tion as before until the crosshead reaches the end of the stroke 
and until the end of the crosshead nearest the end of the guides 
comes back on the return stroke to the line representing one- 
quarter stroke. If the valve corresponding to this end of the 
cylinder has not been released from the hook, adjust the gover- 
nor rod slowly until it is just released. Should the valve be 
released before the crosshead reaches the line, then the governor 
rod will be adjusted slightly so as to cause the valve to be re- 
leased later. The crosshead should then be moved back a trifle, 
then move it forward slowly until it reaches the line, again noting 
at what point the valve is released. In this way the governor rod 
may be adjusted so that the valve will be released by the time 
the crosshead reaches the line. The blocking should now be 
removed from the governor and the safety device inserted so 
that the governor balls will not descend to their lowest position. 

It is always well to put in the starting bar and work the 



15 



wristplate back and forth a few times to see that the hooks 
properly engage the catchblocks. 

After the engine is started and all the parts have reached 
their normal temperature, it is a good plan to connect the indi- 
cator and take a few diagrams, which will indicate the effects ot 
the expansion of the gear upon the action of the valves. It is 
probable a very slight adjustment of the radial rods or the gov- 
ernor rods may be necessary, but if the work has been correctly 
done, as previously described, the adjustment of any of the parts 
will be very slight. The directions for setting the exhaust valves 
to give the proper compression, and for equalizing the points of 
cutoff apply also to the double eccentric Corliss gear, because 
after the eccentrics are once set these adjustments are made by 
means of the radial rods and governor rods the same as with 
the single eccentric gear. 




16 



ADJUSTING THE DOUBLE ECCENTRIC GEAR. 



THE double eccentric Corliss engine when first introduced 
was employed principally in street railway service, where 
the load fluctuates through a wider range than in almost 
any other service with the exception of rolling mills. The use 
of two eccentrics provides the engine with what is commonly 
styled the long range cutoff, because the additional eccentric 
enables the point of cutoff to be extended to approximately 3-4 
stroke, thus giving a period of admission nearly twice as long 
as can be obtained in the single eccentric machine. The advan- 
tages of the long range cutoff are not confined to convenience 
and in enabling the engine to carry very heavy overloads. It is 
well known that with a given initial pressure and piston speed- 
the size of the cylinder may be reduced as the point of cutoff 
occurs later in the stroke. A 12 x 36 Corliss engine in which the 
cutoff occurs at 1-2 stroke when making 117 revolutions per min- 
ute will develop as much power as a 14 x 36 engine making 100 
revolutions per minute in which the cutoff occurs at 3-8 stroke. 
The maximum power of the single eccentric engine is reached 
when the cutoff occurs at about 3-8 stroke; therefore, this style 
of engine must be so proportioned that the heaviest load can be 
carried without interfering with the speed regulation when the 
cutoff occurs not later than 3-8 stroke. The double eccentric 
engine may be proportioned to carry the normal load at 3-8 
cutoff, if so desired, because the maximum power of the engine 
is not attained until the point of cutoff occurs at approximately 
3-4 stroke. It is evident that for equal power and with the same 
percentage of overload to be provided for, the single eccentric 
engine will have a somewhat larger cylinder, which incidentally 
adds to the initial cost of the engine. Taking two engines of 
the same size, one a single eccentric and the other a double eccen- 
tric engine, when the cutoff occurs at, say, 1-4 stroke in each, the 
single eccentric engine will develop 80 per cent of the maximum 
power, while the double eccentric engine will develop only 61 1-4 



17 



per cent of the maximum power attainable, assuming the initial 
pressure, back pressure and piston speed to be the same in both 
cases. 

As a general thing, the advantages of the double eccentric 
engine with reference to the distribution of steam can be more 
clearly understood by considering the limitations of the single 
eccentric gear. When a person clearly understands why the 
point of cutoff with the single eccentric gear cannot be extended 
beyond a certain point, which, as already stated, is approxi- 
mately at 3-8 stroke, he at once understands the more practicable 
method of extending the point of cutoff and is able to compre- 
hend the necessity for the additional gear. The limitations of 
the single eccentric gear may be explained as follows: 
Latest Cutoff in Single Eccentric Engine. 

The limit of cutoff or the latest point of cutoff obtainable in 
the single eccentric engine is illustrated in the accompanying 
drawings. Fig. 6 represents the position of the valves, crank 

fa 





FIG. 6. VALVES AND ECCENTRIC AT THE BEGINNING OF THE STR0E3J. 

and eccentric at the commencement of the stroke. The steam 
valve at the head end of the cylinder has opened the port to the 
extent of the lead, while the eccentric has been turned around 
on the shaft to a point corresponding to the lap and lead of the 
steam valves. When the crank reaches the position shown in 
Fig. 7, the eccentric will have reached the dead center farthest 
from the cylinder and it is evident that any further movement 
of the valves in the same direction is impossible, in other words, 
the valves have reached the extreme limit of their travel. Neg- 



18 



lecting the effect of inertia and recognizing the fact that nothing 
occurs instantaneously in mechanics, the position of the piston 
in Fig. 7 indicates the latest point of cutoff, which, it will be 
noticed, occurs at about 3-8 stroke as long as the governor 
remains operative. 





FIG. 7. ECCENTRIC ON ITS OUTWARD DEAD CENTER. 

A very natural question is, why cannot the valves be given 
less lap and the radial rods shortened, thus permitting the eccen- 
tric to be set more nearly at right angles to the crank ? That, of 
course, can be done, and the radial rods connected to the exhaust 
valve stems can be adjusted to compensate for the new position 
of the eccentric. Fig. 8 represents the cylinder of an engine in 




FIG. 8. SINGLE ECCENTRIC ENGINE— VALVES GIVEN LESS LAP. 

which these changes have been made. Very little difference in 
the position of the parts is noticeable, excepting the position of 



19 



the eccentric, because it is only possible to obtain a slight change 
in this manner even when specially constructed valves are em- 
ployed. 

It must not be forgotten that both the steam and exhaust 
valves must have some lap in order to render them steam tight 
when closed, and that about 1-8 inch lap is as little as can be 





FIG. 9. ECCENTRIC ON THE OUTWARD DEAD CENTER. 

depended on. When the crank, illustrated in Fig. 8, is turned 
to the position indicated in Fig. 9, the eccentric will occupy the 
same position as in Fig. 7, and it will be seen that the same con- 
ditions obtain, viz., the eccentric has reached the dead center 
farthest from the cylinder and the valves have reached their 




'/S//////A y;;/s//Ss>Ss>& 



'SSSSS/SSS/SASSSSSSSSSSSSS, 







W 222 * 





FIG. 10. PISTON AT POINT WHERE COMPRESSION SHOULD BEGIN. 

extreme position during the outward stroke of the piston. The 
position of the piston indicates that the cutoff now occurs at 



20 



about 1-2 stroke, which apparently is an improvement. The 
difficulties with this arrangement are experienced on the return 
stroke and also at the end of the outward stroke. Fig. 10 illus- 
trates the position of the valves and piston when release should 
occur, and it is found to be much too late, which is due to the 
fact that by moving the eccentric back or nearer the cylinder it 
becomes necessary to re-adjust the length of the rods operating- 
the exhaust valves so that the valves may occupy the proper 
position at the commencement of the stroke. This adjustment 
has resulted in the lengthening of the rod at the head end of the 
cylinder and shortening the one at the crank end. Fig. 11 illus- 
trates the conditions obtaining when the piston reaches the 
end of the stroke. 



•■■■■■v: ■■■■■■■.■■■■■■■■■•:■■■■■■■■...:;•■■■. ■■v-^ 




VZ2Z&2Z22ZSBS22Z&B5BZ 





FIG. 11. PISTON AT THE END OF THE OUTWARD STROKE. 

The exhaust valve at the crank end of the cylinder has not 
quite closed the port, indicating that no compression can be 
obtained, and that release at that end of the stroke will take place 
much too early. The exhaust valve at the head end of the cylin- 
der has not yet opened the port, showing that release is too late,, 
and not only that, but it is evident from the position of the eccen- 
tric that the valve cannot open the port wide, hence the port will 
be closed earlier during the return stroke, thus producing too 
much compression. 

Equalizing the Motion. 

When about to set the double eccentric Corliss valve gear, 
the same care should be exercised in getting the gear into proper 
working order — that is, with reference to the lap of the die plate 
on the hooks and catchblocks, seeing that the gear and dashpots 



21 



are clean and free from all unnecessary lost motion, and that the 
dashpot plungers seat properly. Measure the width of the steam 
port which is to be uncovered by the valve, and note it on paper. 
The motion or travel of the valves must be equalized the same 
as with the single eccentric gear so that the two steam valves 
and the two exhaust valves will have equal lengths of travel. The 
method of equalizing the motion of each of the wristplates is pre- 
cisely the same as with the single eccentric gear, therefore, it will 
be unnecessary to repeat the directions. First equalize the travel 
of the steam wristplate, then equalize the travel of the exhaust 
wristplate, marking the extreme positions and the central posi- 
tion of each wristplate, by means of lines on the wristplate hub 
and its support, the same as with the single eccentric gear. 

Adjusting the Position of the Valves. 

The first step, after equalizing the motion of the wristplate, 
is to turn the steam eccentric on the shaft so that the marks 
A and B, on the wristplate hub and stud are exactly in line with 
each other, as shown in Fig. 1 A. Tighten the set screws in the 
steam eccentric or otherwise lock the wristplate firmly in this 
position. If the edges of the valves and ports are not indicated 
by means of lines on the ends of the valves and on the valve seats, 
pull out one valve at a time and put on the marks as described 
in connection with the single eccentric gear. After marking the 
valves and the valve seats and replacing the valves, see that the 
hook has engaged the crank arm and has raised it, as in Fig. 1. 
The marks, in the case of the double eccentric gear, should indi- 
cate that the valve has opened the port about one-fourth of the 
width of the port, which opening will be indicated by the line on 
the end of the valve and on the valve seat being one-fourth of the 
width of the port apart. If the port is opened too wide or not 
wide enough, move the valves by adjusting the right-and-left con- 
nection in the radial rods. Release the wristplate and pick up 
the opposite valve and adjust its position while the wristplate 
is at mid-travel. 

Raise the steam reach rod and move the steam wristplate to 
the end of its travel, which position will be indicated by the 
lines AC or AD, being in line with each other. Next look at the 



22 

hook, which is now in position to engage the catchblock on the 
crank-arm. Lengthen or shorten the dashpot rod until the clear- 
ance between the die plates on the hook and on the catchblock is 
about 1-32 of an inch. Next look at the line on the end of that 
valve and note the lap, which will be indicated by the line on the 
valve being now on the opposite side of the line on the valve seat, 
the distance between these lines being a little less in this direc- 
tion than when the wristplate occupied its central position. If 
the lap is too small, the minimum lap being about 1-8 of an inch, 
adjust the position of the valve by means of the radial rod which 
operates it, afterward adjusting the length of the dashpot rod 
so as to hold the valve in this position. Then give the proper 
clearance between the die plates on the hook and catchblock by 
adjusting the radial rod. Turn the wristplate to the extreme 
position in the opposite direction, and adjust the opposite steam 
valve in precisely the same manner. 

Setting the Steam Eccentric. 

The next step is to place the crank on the dead center, pref- 
erably the one nearest the cylinder. Turn the full part of the 
steam eccentric to its dead center or as far as it will go toward 
the cylinder. Now note the kind of valve gear employed. If 
it is of the style shown in Fig. 12, turn the steam eccentric on the 
shaft in the same direction in which the engine is to run until 
the steam valve at the head end of the cylinder has opened the 
port 1-32 of an inch. Tighten the set screws in the eccentric. 

Measure the distance the wristplate has been moved from its 
extreme position, which distance will be indicated by the lines on 
the wristplate hub and its support. Lay off this distance on the 
wristplate support from the extreme position in the opposite 
direction, and mark it, then turn the wristplate so that the line 
B, on the wristplate hub, comes in line with the one just put on 
the support. The wristplate will now occupy the same position 
that it would if the crank had been turned to the opposite dead 
center. The other steam valve — that is, the valve opposite to the 
one first adjusted — should now have opened the port 1-32 of an 



23 





A A 





24 

inch. Its position may be adjusted, if need be, by means of the 
radial rod. 

Adjusting the Exhaust Valves. 

The next step is to have the exhaust eccentric turned round on 
the shaft until the exhaust wristplate reaches its central position, 
which is indicated by means of the marks A and B on the wrist- 
plate hub and its support. These lines should be exactly in line. 
The lines on the ends of the exhaust valves and on the valve 
seats should first be brought exactly in line with each other for 
speeds up to 90 revolutions a minute, and for higher speeds they 
should be given 1-16 of an inch lap as the trial position. 

The next step is to set the exhaust eccentric. First place the 
crank on the dead center nearest the cylinder. The more con- 
venient starting point in this case is to have the exhaust wrist- 
plate in its central position, and have the full side of the eccen- 
tric on the quarter — that is, either above or below the shaft, de- 
pending on the style of the gear employed. If the tops of the 
valves move toward the exhaust pipe when opening the ports, 
which is generally the case, then the full side of the exhaust ec- 
centric will be placed according to the connections of the reach 
rod with the wristplate. If the exhaust reach rod is connected to 
the wristplate below the center pin on which the wristplate swings 
or oscillates, as shown in Fig. 13, then the full side of the exhaust 
eccentric will be turned to the lower quarter or below the shaft. 
When the reach rod is connected to the wristplate above the cen- 
ter pin, as shown in Fig. 12, then the full side of the eccentric 
will be turned to the upper quarter or above the shaft. Having 
inspected the connection of the reach rod to the wristplate, turn 
the eccentric to the approximate quarter, hook on the reach rod, 
set the wristplate in its central position by moving the eccentric 
and secure it in this position in such manner that the crank can 
be turned without moving the exhaust gear. 

Adjusting the Exhaust Valves for Compression. 

Next lay off on the guides the compression marks H and F, 
Fig. 4, in the manner described for the single eccentric gear. 
Then have the crank turned in the same direction in which the 
engine is to run until the end of the crosshead nearly reaches the 
opposite end of the stroke and comes up to the line on the guide. 



25 



Loosen the exhaust valve gear, and turn the exhaust eccentric so 
that the valve which is now nearest the piston will just close the 
exhaust port, or in other words, so that it is in position to open 
the port, the edges being in line. Tighten the eccentric to the 
shaft, then turn the crank in the same direction as before until 
the crosshead conies back on the return stroke and reaches the 
line on the guide. The opposite exhaust valve should now have 
just closed the port. If this valve needs moving it may be ad- 
justed by means of the right-and-left connection in the radial 
rod. Compression will then begin when the piston reaches the 
points in the stroke corresponding to the position which the 
crosshead now occupies. That completes the setting of the ec- 
centrics of the double eccentric Corliss gear, and all that remains- 
is to see that the governor prevents the hooks from engaging the 
catchblocks when the governor balls are in their lowest and high- 
est positions, and to equalize the points of cutoff, which is accom- 
plished in precisely the same manner as with the single Corliss; 
gear previously described. 

Styles of Corliss Gears. 

Referring to the diagrams, Figs. 12 and 15, it will be seen 
that there are two styles of Corliss valve gear employing the 
wristplate ; the principal difference is that in one style the radial 
rods are connected near the top of the wristplate, and in the 
other they are connected at the sides. It will also be noticed that, 
in Fig. 12, the top of the wristplate moves in the same direction 
as the piston when opening the steam port at the head end of the 
cylinder, but in Fig. 15 the top of the wristplate moves in the op- 
posite direction to the piston when opening the same port. It is- 
evident, therefore, that some change in the position of the steam 
eccentric relative to the crank will be necessary in order to pro- 
duce the two motions, When the gear shown in Fig. 15 is em- 
ployed the eccentric should preferably be placed at the opposite' 
dead center to the crank as the starting point and then turned in 
the same direction in which the engine is to run until the valve 
at the head end of the cylinder opens the port 1-32 of an inch. 

By inspecting Fig. 17 closely it will be seen that, when the 
style of gear shown in the drawing is employed and when the 
reach rod is connected below the wristplate stud or bearing, in- 



26 







uo 



27 



stead of above it, the motion of the steam eccentric when opening 
the port at the head end of the cylinder will be the same as in 
Figs. 1 and 12, and hence the eccentric will start from the same 
dead center and will be turned in the same direction as in Figs. 
1 and 12. The tops of the steam valves in most Corliss engines 
turn toward the middle of the cylinder or toward the steam pipe 
when admitting steam, and when this is the case the eccentric in 
the two styles of Corliss gears will be turned in the directions, 
and from the starting points, indicated in the diagrams, Figs. 
12 to 17, provided the piston in all cases is at the head end of the 
cylinder as a starting point, when turning the steam eccentric, 
and the crosshead at the compression line on the guides when 
turning the exhaust eccentric. It will be understood that in 
cases where the direction of rotation is reversed and is " under " 
instead of ' ' over, ' ' the directions in which to turn the eccentrics 
and the starting points for the eccentrics will be reversed from 
those indicated in the diagrams and drawings. 

The diagrams, Figs. 12 to 17, represent the two styles of 
Corliss valve gear employing the wristplate motion, the gears 
being in the position they occupy when properly set and with 
the crank on the dead center nearest the cylinder. In the dia- 
grams, Figs. 12 to 17, the steam eccentric and gear are marked 
8, and the exhaust eccentric gear, E. For convenience the crank 
in all the diagrams is represented on the dead center nearest the 
cylinder, and the direction in which it is to turn is shown by the 
large arrow, this direction being known as "over," or running 
over. The starting point which will be found least confusing 
when moving the eccentrics is indicated by dotted lines. In Fig. 
14, the steam eccentric is first placed on the center toward the 
cylinder, and the exhaust eccentric in the opposite position. 
Then when setting the steam eccentric it is moved toward the 
right to the position indicated by the full center line 8, the 
smaller arrow showing the direction in which the eccentric is 
moved. As previously described, the exhaust eccentric is not set 
while the crank occupies the dead center, but instead, when it is 
at a point corresponding to the point in the stroke where com- 
pression begins. But the exhaust eccentric is moved from the 
starting point in the direction indicated by the small arrow. 



28 







29 



Fig. 15 shows another style of gear. With this gear the 
starting point for the steam eccentric is on the center farthest 
from the cylinder, while the exhaust eccentric is started from the 
lower quarter. In Fig. 16 the starting point for the steam eccen- 
tric is on the center farthest from the cylinder, and for the ex- 
haust eccentric, the opposite center. In Fig. 17 the starting 
points are just the reverse of those in Fig. 16. The object in 
having these starting points is merely to simplify the work for 
persons who are not thoroughly familiar with the different styles 
of Corliss gear. The starting points can be located at a glance 
and when once the eccentric is in the proper starting position, it 
needs but a slight movement to bring it into the correct and final 
position. The causes for the different positions of the eccentrics 
are found at the wristplates, the connections to the valves being 
such that the motions indicated by the small arrow must be ob- 
tained in order to properly open the valves. The diagrams rep- 
resent about all the different arrangements for rocker arms, 
wristplates and radial rods ordinarily employed. With Corliss 
gears in which no wristplates are employed, the steam eccentric 
is set in the same manner as the single eccentric gear shown in 
Fig. 1, while the exhaust eccentric is set in the same manner 
as that illustrated in Figs. 13 and 14, 15 and 17. 

To Place the Crank on the Exact Dead Center. 

To place the crank on the exact dead center, put a punch mark 
or a line A, Fig. 18, on the guide near the crosshead, and from 
1-4 to 1-2 inch from the end of the crosshead travel. Procure a 
strip of wood about 3 feet long, then sharpen a nail to a fine 
point and drive it through the strip at one end. Have the crank 
turned in the same direction in which the engine is to run until 
the crosshead reaches the line near the end of the guide. Now 
place the strip in a vertical position close to the crank disc or 
the rim of the flywheel, and with the point of the nail make a 
line C near the edge. Have the crank turned past the center, as 
indicated by the dotted lines, and until the same end of the cross- 
head comes back to the line A near the end of the guide. Place 
the strip of wood in the same position as before and with the 
point of the nail make another line B, near the edge of the disc 
or flywheel. 



30 



Take a pair of dividers and find the center between the two 
marks on the disc or wheel. At this point make a third mark D. 
Then have the crank turned slowly until the middle mark D 
reaches the point of the nail, the strip of wood being placed in 
the same position as before. The crank will then be on the 




FIG. 18. PLACING THE CRANK ON THE DEAD CENTER. 

exact dead center. The dead center at the opposite end of the 
stroke is found in the same manner. 

Adjusting the Cutoff for the Average Load. 

Probably it has been noticed by persons in charge of Corliss 
engines that when the cutoff is equalized when taking steam, say, 
one-eighth stroke, it will not be equal when the cutoff occurs at 
three-eighths stroke. It is practically impossible to obtain an 
exactly equal cutoff at all points in the stroke, especially in en- 
gines fitted with the long-range cutoff, in which the period of 
admission frequently varies from zero to three-fourths stroke. 
This is due to the fact that the angle between the center line 
of the actuating arm on the governor columns and the cen- 
ter line of the governor rod is not the same when cutoff takes 
place at one-eighth and three-eighths stroke. Referring to 
Fig. 19, it will be seen that the actuating arm is shown in its 
middle position and that the angle between it and the gov- 
ernor rod is approximately a right angle. Under these cir- 
cumstances the motion imparted to the governor rod will be 
equal to the motion of the arm, but if the angle be changed from 
a right angle to an acute or obtuse angle, the motion imparted 
to the governor rod will not be exactly the same as that of the 



31 



actuating arm. Therefore, if the cutoff be exactly equalized 
when the actuating arm occupies its midtravel position, as in 
Fig. 19, when the load becomes such as to cause it to* occupy 
either of the positions indicated by the black dots at the ends of 
the arcs representing its extreme travel, the cutoff will not be 
exactly equal. For this reason if the engine carries a fairly 
steady load for the greater part of the time it is well to raise 
the governor balls to their usual running position when equaliz- 




FIG. 19. GOVERNOR ARMS AT MID-TRAVEL. 

ing the cutoff. While the differences in points of cutoff due to 
the governor occupying its extreme positions are slight and for 
that reason have an unimportant influence on the economy of 
the engine, nevertheless this discrepancy in the steam distribu- 
tion is frequently aggravated by a badly working dashpot, which 
allows the valve too much time in closing, thus increasing the 
period of admission considerably beyond that due merely to the 
action of the releasing gear. This is apt to cause a badly dis- 
torted diagram and to lead an engineer to believe that the gear 
is considerably out of proper adjustment. The average or usual 
height of the governor can be obtained by marking the column 
below the central weight when the engine is working under the 
average load, then raising the weight up to this line before 
equalizing the cutoff. The engine will then cut off equally under 
average conditions of load and the maximum economy obtained 
under all loads. 




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